Export files referred in other place, otherwise running in tensorflow codebase: (#25712)

bazel query "deps(//external:local_config_python)" shows error like this:

ERROR: /home/mingzhao/g/tensorflow/WORKSPACE:19:14: every rule of type python_configure implicitly depends upon the target '@com_github_grpc_grpc//third_party/py:BUILD.tpl', but this target could not be found because of: no such target '@com_github_grpc_grpc//third_party/py:BUILD.tpl': target 'BUILD.tpl' not declared in package 'third_party/py'; however, a source file of this name exists.  (Perhaps add 'exports_files(["BUILD.tpl"])' to third_party/py/BUILD?) defined by /home/mingzhao/.cache/bazel/_bazel_mingzhao/b8418b6d51a5e225bcc1359ca8cb25de/external/com_github_grpc_grpc/third_party/py/BUILD
ERROR: /home/mingzhao/g/tensorflow/WORKSPACE:19:14: every rule of type python_configure implicitly depends upon the target '@com_github_grpc_grpc//third_party/py:variety.tpl', but this target could not be found because of: no such target '@com_github_grpc_grpc//third_party/py:variety.tpl': target 'variety.tpl' not declared in package 'third_party/py'; however, a source file of this name exists.  (Perhaps add 'exports_files(["variety.tpl"])' to third_party/py/BUILD?) defined by /home/mingzhao/.cache/bazel/_bazel_mingzhao/b8418b6d51a5e225bcc1359ca8cb25de/external/com_github_grpc_grpc/third_party/py/BUILD
ERROR: Evaluation of query "deps(//external:local_config_python)" failed: errors were encountered while computing transitive closure

After this fix:

 bazel query "deps(//external:local_config_python)"

//external:local_config_python
@com_github_grpc_grpc//third_party/py:variety.tpl
@com_github_grpc_grpc//third_party/py:BUILD.tpl

third_party/py/BUILD

@@ -0,0 +1,5 @@
+exports_files([
+    "BUILD.tpl",
+    "python_configure.bzl",
+    "variety.tpl"
+])